The aim of this retrospective chart-review study was to investigate the relationship between delayed commencement of clozapine and the level of response in treatment-resistant schizophrenia (TRS). We included 162 patients with schizophrenia who used clozapine. The mean delay until starting clozapine after fulfillment of the TRS criteria was 29 months. The delay was shorter in those who gained benefit from clozapine (P=0.04), those who were treated in a specialized psychosis outpatient unit (P=0.01), and in men (P=0.009), and it correlated with age (P<0.001). The delay in starting clozapine and the maximum clozapine dose were independent contributors toward the response to clozapine in the logistic regression analysis. Moreover, of those who gained considerable benefit from clozapine, the patients were younger (P=0.01), the duration of illness before clozapine treatment was shorter (P=0.001), and the numbers of adequate antipsychotic trials before the use of clozapine were fewer (P=0.05). Our findings suggest that efforts aimed at reducing the delay for starting clozapine may increase the effectiveness of clozapine in TRS.